1. Field of the Invention
The present invention relates to an image recording apparatus and, more particularly, to an image recording apparatus for forming a color image by printing coloring agents of a plurality of colors.
2. Related Background Art
Hitherto, an ink jet recording apparatus to form an image by spouting ink from nozzles each having a small diameter has been well known. The ink jet recording apparatus is widely used as a color image recording apparatus since a color image can be easily obtained by overlapping printing inks of a plurality of colors.
FIG. 4 shows a scanning method of ink jet heads in the case of obtaining a color image by overlapping the inks of three colors of yellow, magenta, and cyan.
In the diagram, multi-nozzle heads lA, lB, and 1C are arranged at a distance d from each other and are scanned on a recording sheet 3 at a velocity v in the direction of an arrow 4 while spouting ink from orifices 2. The head lA is used for the yellow ink. The head lB is used for the magenta ink. The head 1C is used for the cyan ink. These inks are printed on the recording sheet 3 in accordance with the order of yellow, magenta, and cyan.
FIG. 5 is a block diagram for image signal processes of such an ink jet recording apparatus.
Input signals 5a to 5c indicative of image densities of three colors of yellow, magenta, and cyan are supplied to a color process unit 6 and are subjected to color processes such as a masking process and the like. Thereafter, the color processed signals are input to a gradation correction unit 7 and are Y-corrected. After completion of the correction, the yellow signal among the three color signals is directly sent to a recording head 9A. However, the magenta and cyan signals are first stored into buffers 8A and 8B and thereafter, they are delayed by the time corresponding to a distance d in the scanning direction of the recording heads. Namely, the magenta signal is delayed by the time of d/v and the cyan signal is delayed by the time of 2d/v. The delayed magenta and cyan signals are sent to heads 9B and 9C.
Thus, the inks of the respective colors of yellow, magenta, and cyan are printed at the same position on the recording sheet 3 and a color image is reproduced.
In such an ink jet recording apparatus, when the heads are driven, beat is generated therefrom and, by continuing the printing operation, the temperatures of the heads increase. In general, although the viscosity of ink decreases with an increase in temperature, the emission amount of ink from the head increases with a decrease in viscosity.
On the other hand, a stained amount of ink in the recording sheet also increases as the viscosity of ink is low. Therefore, the dot area on the recording sheet is enlarged by the temperature increase of the head and the image density raises.
FIG. 6 is a diagram for explaining such a situation and shows that the .gamma. characteristic changes by a change in temperature of the head.
In this manner, the image density and .gamma. characteristic change by an increase in temperature of the head. On the other hand, as shown in FIG. 4 in an apparatus such that a color image is formed by spouting ink of different colors from a plurality of heads, an increase ratio of the temperature of each head is not uniform. There is a difference of the increase in temperature of each head in dependence on original documents. For example, in the case of an image having a blue sky as a background image, the temperature of the cyan head particularly increases. In the case of an image having an evening sun as a background image, the increase in temperature of the cyan head will be minimal. In such a case, there occurs a difference of the .gamma. characteristic of each color, so that the color balances before and after the temperature increases are different and there is a drawback such that the color reproduction cannot be stably performed.
Particularly, in an apparatus such that the ink is heated by a heater and is emitted by the thermal energy, the foregoing inconvenience is remarkable.
To solve the foregoing drawbacks, the same applicant as the present patent application has proposed techniques such that amplification factors and the like of input color signals are variably controlled by detecting an ambient temperature and humidity in Japanese Unexamined Patent Publication (Kokai) Nos. 13855/1982, 13856/1982, 13857/1982, and 13445/1982. However, in these techniques, since the temperature of the head itself is not detected, it is difficult to accurately control a color image.